Deep-Water Volcaniclastic Layers in the Late Messinian Apennines Foreland Basin Unravel the First Calc-Alkaline Rhyolitic Eruption in the Central Italy Magmatic System

Abstract

A package of upper Messinian volcaniclastic layers (UMVLs), exposed in the deep-water foreland basin system of the central Apennines (Italy), is the volcanic product of a rhyolitic eruption dated to 5.5 Ma. These UMVLs are an important marker for stratigraphic correlations along the central Apennines foreland basin system, but their source is still debated and poorly understood. Italian Plio-Quaternary volcanism exhibits significant petrological and geochemical variability, causing debate over magma genesis and differentiation. Investigating the magmatic evolution of central Italy is crucial for understanding one of the most complex geodynamic settings on Earth. The first evidence of efficient magma differentiation, producing eruptible calc-alkaline rhyolitic magmas, is the San Vincenzo eruption at 4.41 Ma. Our sedimentological and petrological analyses of UMVL exposures indicate a possible volcanic source in the northeastern Tuscany Magmatic Province. This discovery implies a developed transcrustal magma reservoir system and suggests that efficient magma differentiation capable of producing eruptible calc-alkaline rhyolitic magma occurred about one million years earlier than the San Vincenzo eruption, marking these UMVLs as the first rhyolitic eruption associated with Italian Plio-Quaternary volcanism.

1. Introduction

In western central Italy (Tuscany Magmatic Province, TMP), the formation of transcrustal magma reservoirs was initiated by crustal anatexis of the Elba intrusions at 8.5 Ma [1] and promoted magma migration and differentiation within the crust. Capraia volcano mafic and intermediate magmas, between 7.6 and 4.6 Ma, denote the first evidence of arc volcanism. Subsequent calc-alkaline rhyolitic arc magmas of the 4.4 Ma San Vincenzo eruption and the 4 Ma Ponza volcanism demonstrate progressive magma differentiation over a 3-million-year time span. In addition, upper Messinian volcaniclastic layers (UMVLs), which form an important marker for regional-scale stratigraphic correlations across the deep-water foreland basin system of the central Apennines [2,3], provide evidence of a rhyolitic eruption dated to 5.5 Ma [4,5,6]. These volcanic products result from the explosive eruption of a calc-alkaline rhyolitic magma related to arc volcanism [7,8,9,10,11] and constitute distal eruptive deposits that represent the only preserved record of volcanic activity, as the original volcanic edifices have been lost. Although several recent studies have investigated UMVLs from a petrological and geochemical perspective, their origin remains debated. UMVLs are defined as the products of secondary monomagmatic volcaniclastic turbidites [9,12,13,14], suggesting a rifting–spreading episode from an ancient volcanic arc in the southern Tyrrhenian as a possible source. By contrast, other studies have interpreted UMVLs as the products of a distal fallout from multiple explosive eruptions [10,11], suggesting the Carpathian-Pannonian arc convergent setting as a possible magmatic source. Constraining the source of the UMVL eruption is important to better understand the development of transcrustal magma reservoirs in the central Italy magmatic systems and the magma differentiation (from basaltic to rhyolitic magmas) related to arc volcanism [15]. In particular, if UMVLs are related to Italian arc volcanism, it would be useful to understand (i) the first rhyolitic eruption produced by Italian Plio-Quaternary arc volcanism and (ii) the differentiation from mafic to silicic magmas, resulting in eruptible rhyolitic magmas, which occurred over approximately 2.1 million years (from the Capraia eruptions at 7.6 Ma to the UMVLs at 5.5 Ma).

To investigate the source of the UMVL eruption, we performed macroscopic- and microscopic-scale sedimentological and petrographic analyses of the volcanic products collected at Amandola (AM-2, Figure 1B) and Camporotondo di Fiastrone (CF, Figure 1C), two localities in central Italy preserving the thickest deposits of the UMVL eruption. These data provide new insights into the processes by which subaqueous density flows deposited sediments to form UMVLs and shed new light on the location of the magmatic source.

2. Materials and Methods

To obtain a detailed sedimentological and petrographic characterization of the UMVLs, volcaniclastic samples from AM-2 and CF were studied at different scales. Macroscopic inspections were conducted both in the field and on polished slabs, whereas qualitative textural tephra particle characterization was undertaken through image analyses [16] of high-resolution digital photographs of thin sections. Petrographic and chemical compositions were determined by (i) optical microscopy; (ii) X-ray diffraction (XRD) performed using an automated PHILIPS diffractometer equipped with diffracted-beam soller slits, 1° divergence slits, 0.1 mm receiving slits, and a graphite diffracted-beam monochromator (at Unicam, Camerino, Italy); (iii) a scanning electron microscope FE-SEM LEO 1525 Zeiss using back-scattered electron detectors AsB (angle-selective backscattered detector), an Inlens secondary electron detector, and an EDX detector Bruker Quantax 200 (equipped with a X-Flash 410 detector) at Unicam, Camerino, Italy; and (iv) electron probe micro-analyzer (EPMA) analyses using a Cameca SX100 at Centro Nazionale di Ricerca of Firenze (Sesto Fiorentino, Italy) and a JEOL JXA-8530F field emission electron microprobe at the Photon Science Institute, University of Manchester.

3. Results

3.1. Macroscopic Description of the Volcaniclastic Deposits

The UMVLs are 1.9 m and 2.35 m thick at AM-2 and CF, respectively (Figure 2A). Both sections are composed of 22 event beds, featuring sedimentological characteristics typical of submarine density flow deposits. Individual event beds, ranging in thickness from 5 to 30 cm, display normal grading and sharply defined bases, commonly with load structures and flames (Figure 2B). Internally, they are composed of one or more of the intervals proposed by [17] in their classification scheme for subaqueous density flow deposits (Figure 2B–D). The most complete set of intervals includes both high- and low-density turbidity current deposits and comprises (i) coarse, sand-sized glass particles with centimeter-scale spaced (stepped) planar lamination displaying internal inverse grading (T_B-3) deposited incrementally by high-density turbidity currents, including imbricated pumice particles up to 1 cm in size (Figure 3A), rare rip-up mud clasts as much as ~1 cm in diameter (Figure 3B), and foraminifera shells (Figure 4F) picked up from the seafloor during transport; (ii) a massive, poorly graded T_A interval recording progressive layer-by-layer deposition by high-density turbidity currents; (iii) a fine-scale planar-laminated interval (Figure 3C), formed incrementally by the migration of low-amplitude bed waves beneath dilute flows (T_B-1) or by the repeated collapse of traction carpets beneath high-density turbidity currents (T_B-2); (iv) a ripple cross-laminated and convoluted T_C interval (Figure 2C and Figure 3D,E); (v) an interval of poorly developed planar laminae in silt-grade volcanic glass (T_D); and (vi) massive mud-grade volcanic glass (T_E), (Figure 3F). The main paleocurrent directions indicate a north–northeast provenance of the sediment gravity flows.

3.2. Microscopic and Petrographic Characterization

XRD, SEM, and EPMA analyses (Figure 4 and Figure 5) indicate that the UMVLs are composed of 80% glass shards and pumice fragments with a calc-alkaline rhyolitic composition (

Table 1. Microprobe analysis of the matrix glass in the cm-sized pumices.

	SiO2 (wt%)	TiO2 (wt%)	Al2O3 (wt%)	FeO (wt%)	MnO (wt%)	MgO (wt%)	CaO (wt%)	Na2O (wt%)	K2O (wt%)	P2O5 (wt%)	Total (wt%)
VL-01	77.70	0.12	13.60	1.11	0.05	0.07	0.79	4.29	2.27	0.01	96.15
VL-02	77.37	0.08	13.55	1.07	0.00	0.06	0.78	4.60	2.48	0.01	96.36
VL-03	77.28	0.11	13.58	1.18	0.02	0.09	0.76	4.57	2.42	0.00	96.96
VL-04	76.75	0.12	13.90	1.38	0.02	0.07	0.84	4.48	2.43	0.00	96.33
VL-05	76.70	0.09	14.03	1.38	0.03	0.08	0.84	4.31	2.52	0.02	95.94
VL-06	76.74	0.11	13.98	1.34	0.02	0.07	0.84	4.41	2.44	0.05	96.12
VL-07	78.11	0.06	13.46	1.12	0.02	0.04	0.78	4.23	2.19	0.00	95.76
VL-08	77.82	0.11	13.44	1.22	0.07	0.06	0.79	4.46	2.06	0.00	96.27

Note: The oxide values are normalized, whilst the total reported in the table is the original obtained from EPMA analysis.

and Figure 5) and 20% several other mineral phases. XRD analyses reveal that the UMVL tephra is composed of 38% feldspar, 26% quartz, 14.8% biotite, and 21.2% calcite (also constituting the UMVL matrix) (Figure 4B). The glass shards show cuspate, microvesicular, and platy shapes (Figure 4C), a typical product of explosive magmatic eruption [18]. The petrographic and geochemical data are in close agreement with those reported by [5,7,8,9,10,11]; nevertheless, the novel observation of our study regards highly vesiculated pumice clasts, up to 1.8 cm large, which were encountered in the CF beds (Figure 4D). The pumices had very elongated (~200 µm) and flattened vesicles, parallel to the pumice clast elongation (Figure 4D,E). This type of deformation and alignment is characteristic of “tube” pumice structures [12,19,20,21], which are considered reliable indicators for reconstructing the strain history of ascending magmas’ prior fragmentation, as their geometry reflects the stress state induced by a complex intra-conduit mechanism [19,20]. Optical microscopy also reveals the presence of planktonic foraminifera (Figure 4F).

4. Discussion and Conclusions

The sedimentary structures in the UMVLs exposed at AM-2 and CF and the presence of small mud rip-up clasts, typical of high-density turbidity currents, attest to the emplacement of the volcaniclastic beds by submarine high-density flows. Based on petrographic data, UMVLs are defined as secondary volcaniclastic turbidites, i.e., composed of more than 50% of volcanically derived particles [12,14,23]. Volcaniclastic turbidites can directly link to volcanic activity [24] and result from pyroclastic flows interacting with water [25] in various volcanic settings, such as (i) during the final stage of a submarine eruption, when reduced fallout from the eruptive column becomes insufficient to sustain primary submarine pyroclastic flows triggering repeated turbidity currents [25,26,27], and (ii) when the subaerial pyroclastic flow entering a subaqueous environment rapidly cools upon reaching the sea, spreading underwater for several kilometers [23], until mixing with ambient water transforms it into high-density turbidity currents [28]. However, without land-correlated volcanic products, establishing whether deep-water UMVLs originated from a submarine or subaerial vent is challenging [29,30]. The geochemical homogeneity observed across the volcaniclastic beds composing the UMVLs indicates that they were deposited during a single volcanic event from the same magmatic source. This compositional uniformity serves as a fingerprint of the volcanic system, as magma chemistry typically varies between distinct volcanic events [9]. Since each bed is described as the product of a single subaqueous density flow, multiple depositional events have been generated by a single volcanic event. These observations constrain the timing of deposition, suggesting that the density currents were generated during different eruptions of a single volcanic event, or resulted from distinct emplacement regimes of the same eruption [31]. In either case, the depositional succession reflects well-defined temporal stages of the eruption. In addition, the lack of hemipelagic interbeds supports the interpretation that the individual beds of the UMVLs were emplaced over a short period of time (i.e., days or weeks). The highly vesiculated rhyolitic pumice clasts (~2 cm) occurring in the T_B-3 intervals of the CF section, representing juvenile material entrained in turbidity currents, are key indicators of conduit ascent processes, e.g., [21]. Size, shape, and distribution of vesicles provide information on strain indicators and pre-fragmentation stress conditions [32]. Stretched bubbles in tube pumices represent a well-preserved magmatic “strain marker” [10,21] and are referred to as the product of induced simple shear along the conduit walls during the last-stage deformation of vesicular magma, where vesicles undergo extensive shear and deformation before eruption [14,19,21,33]. For these reasons, the stretching vesicles rule out the hypothesis of lithostatic compaction and remobilization, supporting the interpretation of UMVL deposits as multiple “secondary monomagmatic volcaniclastic turbidites” [9,12,14]. This interpretation is also sustained by the modern analogue provided by Facies 3 described by [34,35] for the Soufriére Hills Volcano. Similar to UMVL deposits, Facies 3 is characterized by moderately to well-sorted sand- and silt-sized volcaniclastic particles that compose thin laterally continuous sheet-like beds deposited by high-density turbidity currents. Furthermore, the sedimentary features observed in UMVLs align with the massive to planar-laminated intervals, ripple cross-lamination, and clear evidence of tractional structures in Facies 3. Both deposits show well-developed vertical grading, produced from the layer-by-layer deposition of turbidity currents. These similarities strengthen the interpretation proposed here, in which UMVL beds were emplaced by far-reaching turbidity currents generated by pyroclastic flows entering a subaqueous environment, as observed in the analogous case at Montserrat [34,35]. Considering that the underwater spreading capability of dense pyroclastic flows is a few tens of kilometers [23,30] and the CF deposits are rich in coarse juvenile clasts (Figure 3B), the Carpathian-Pannonian arc provenance proposed (700–1000 km of distance) by [11] is implausible. The coarse ash deposits and centimeter-sized pumices are unlikely to have traveled for such a long distance as ballistic objects. This implies a closer magmatic source to the deposit area. The petrographic analysis conducted by [36] on siliciclastic sediments deposited both above and below the UMVLs indicates a sedimentary contribution from localized regions of the adjacent Apennine thrust belt. These sediments entered the basin from an apical position, with a northern source and with paleocurrents indicating flow from the north–northwest. Furthermore, immobile element analyses [11] on UMVL products indicate volcanism related to a convergent tectonic setting and volcanic arc magmatism [9]. Here, we propose a new interpretation, suggesting that the source of the UMVLs is linked to the Tuscany Magmatic Province (14–0.2 Ma), which was responsible for the formation of several transcrustal magma reservoirs [37] and magma differentiation [1] within a tectonic setting experiencing compression during the late Oligocene–Middle Miocene time [38,39] (Figure 6). The possible source position of the UMVLs is identified in this work, based on the following evidence: (i) paleocurrents data, suggesting a south–southeastward slope gradient along the roughly NNW–SSE elongated foredeep; (ii) petrographic analyses on siliciclastic sediments enclosing the UMVLs [36], indicating a sediment supply from the Apennine orogenic wedge, which was emerging at that time [38]; (iii) the occurrence of coarse-grained pumices in the Calcinelli (CA) [40], Maccarone (M) [9], CF (this work), and Colle Gallo (CG) [7] sections; and (iv) the fact that spaced planar lamination (T_B-3) commonly occurs directly beneath massive T_A intervals in proximal settings, [17] and references therein, relative to the point of current flow initiation, clearly contrasting with the distal fallout interpretation proposed by [11] for deposits from Carpathian-Pannonian arc convergent setting. Furthermore, the reconstructed Messinian back-arc/volcanic arc–wedge-top boundary (Figure 6) delineates the boundaries of the potential source area, as the UMVLs are related to arc volcanism, [11] and reference therein. These boundaries led to the placement of the UMVL source in the northeastern portion of the Tuscany Magmatic Province [37], providing a margin to account for Messinian evolution, given that it migrated eastward in conjunction with the movement of the thrusts system [38]. From this position, the erupted material spread northeastward and entered the foreland basin system as gravity-driven currents from an apical position, a geographic point located in the northern part of the basin, north of the Campea exposure, following the dominant sediment dispersal pathways of the time [38]. This transport occurred within the deep-water foreland basin system of the central Apennines, whose southeastward slope gradient is indicated by the paleo-flow data [36] and the NNW–SSE axial orientation [41]. The observed north to south variations in outcrop thickness do not necessarily contradict a north-to-south dispersal trend. The deposit geometry is strongly influenced by the basin morphology and the main flow axis. As in the modern example at Montserrat [31,35], thicker and coarser-grained deposits occur along the main axial flow path, whereas deposits become thinner and finer towards the lateral margins. The present-day outcrop belt of the UMVLs intersects the original axial-to-marginal position of the basin in an irregular manner, rather than strictly along the flow direction, resulting in uneven thickness variations along the north–south trend, i.e., thicker when exposures are placed in more axial positions and thinner when located closer to the basin margin. The results of this study shed new light on rhyolitic volcanism related to subduction processes at 5.5 Ma, contributing to the record of the Neogene–Quaternary volcanism of the Italian region and widely considered to be one of the most complex geodynamic settings on Earth [37]. The UMVLs demonstrate that efficient magma differentiation, producing calc-alkaline rhyolitic eruptions in central Italy, occurred at least one million years earlier than the San Vincenzo Rhyolite of the Campiglia Marittima system (4.41 ± 0.04 Ma), which is considered to be the most ancient evidence of calc-alkaline rhyolitic magmas in the Tuscany Magmatic Province [39].